This invention relates to tile fabrication of an electrochemical cell, and, more particularly, to the preparation of a porous separator and the use of the separator in the cell.
Rechargeable electrochemical storage cells or batteries are electrochemical devices for storing and retaining an electrical charge and later delivering that charge for useful power. Familiar examples of the rechargeable cell are the lead-acid cell used in automobiles and the nickel-cadmium cell used in various portable electronic devices. Other, less familiar types of cells include nickel-hydrogen and nickel-metal hydride cells.
Many advanced electrochemical cells are constructed with sets of thin, sheetlike anodes and cathodes. These electrodes are soaked or immersed in an electrolyte of the cell. To prevent the electrodes from touching and shorting, a porous, sheetlike separator is placed between the pales of anodes and cathodes. The electrolyte is infiltrated into the pores of the separator structure to ensure an ionic conductive path between the pairs of electrodes.
An example of a separator material used in advanced electrochemical cells such as the nickel-cadmium cell is porous zirconium oxide (zirconia) fabric. To improve the mechanical strength of the otherwise-fragile zirconium oxide fabric and ensure long-life stable operation off the cell, the zirconium oxide fabric is impregnated with a water insoluble polymeric material. Such materials include, for example, polybenzimidazole or polysulfone, which are chemically stable and wettable in the electrolyte, which is typically a potassium hydroxide solution. The cell is then assembled using this separator material.
In conventional practice, the separator is prepared by dissolving the polymer into a water-miscible organic solvent such as N,N-dimethylacetamide, and then soaking small sheets of the zirconium oxide fabric in the solution. The sheets are removed from the solution and blotted to remove the excess amount of solution. The sheets are next immersed into deionized water to precipitate the polymer onto the fabric and to remove the solvent. In a typical implementation of this approach, a small sheet (typically 4 inches by 8 inches, as limited by the mechanical strength of the fabric) of the polymer-solution soaked zirconium oxide fabric is immersed into a pool of several gallons of deionized water, resulting in a large amount of solvent-contaminated water for each square foot of processed separator. This procedure is described more fully in U.S. Pat. No. 4,293,347, whose disclosure is incorporated by reference. Because these steps are performed manually, the cost of the separator can be as high as about 30 percent of the total cost of the cell, and there is a relatively large amount of waste material such as contaminated water to be disposed of.
There is always a need for improved fabrication techniques for producing such cells at lower cost. The present invention fulfills this need, and provides related advantages.